Generally, concerning image forming apparatuses using an electro-photographing method, such image forming apparatuses are well known which incorporate an LED print head (hereinafter referred to as an LPH) featuring a light source structured of an LED array formed of a plurality of LEDs, and which expose image information onto a photo-conductive drum (which serves as an image carrier) to form a latent image. The LPH concentrates light rays using an optical system, generally called a Selfoc lens which is a convergent rod lens array, so that the LPH exhibits high resolution. However, if the distance between the LPH and the photo-conductive drum changes, the exposed image becomes out of focus, and the resolution decreases. Further, in color image forming apparatus in which a plurality of mono-color images are superposed, color unevenness occurs, which is a major problem.
In order to obtain preferable images, the positional accuracy between the LPH and a focusing plate of the photo-conductive drum must be within ±0.05 mm. Due to this, the focus adjustment was very difficult to achieve for an LPH incorporating a plurality of aligned LEDs.
In order to overcome the above problem, the unexamined Japanese Patent Application Publication No. JP2001-113,763 discloses a technology in which a correction value for print head assembling error is memorized when a head shading correction value is measured and calculated, and to output normal images, when a head shading table is set, referring to the difference between the then correction value of the print head assembling error and the correction value of the print head assembling error memorized when the head shading correction value was measured, an image element on which the shading correction is to be conducted is shifted by a changed amount of the print head.
Further, the unexamined Japanese Patent Application Publication No. 2001-125,347 discloses in which after a line-image pattern is formed on the photoconductive drum as an electro-static latent toner image, the line width of the toner image of the line-image pattern formed on the photoconductive drum is detected, and a print head position adjusting mechanism is controlled based on the detected line width of the toner line-image pattern, whereby the position in the optical axial direction of the LED print head is adjusted so that the line width of the detected toner image becomes within a predetermined value.
In the former Patent Publication No. 2001-113,763, even when the print head assembling error changes due to an exchange of the print head or any vibration of the apparatus, each print head element can be correctively referred to a correction table for the density unevenness, so that the head shading correction is effectively conducted on each print element. Accordingly, correction for the density unevenness is improved.
However, since only the amount of exposure light is adjusted as above, the defocusing problem cannot be overcome, which does not improve any decrease of the resolution. Reduction of the image quality due to the uneven density is barely prevented.
In the previous Patent Publication No. 2001-125,347, after the width of the line-image pattern formed on the photoconductive drum is detected, the position of the LPH in the optical axial direction is adjusted based on the detected line width. However, any decline of the LPH in the longitudinal direction of the LEDs, that is, a decline of the LPH in the axial direction of the photoconductive drum, is not adjusted, and further requires a line-width detecting mechanism, which results in a larger apparatus, and an increase of the production cost. Further, since the line width depends upon the image forming condition, such as the exposure amount, and the developing bias, it is not clear whether optimum focusing is obtained by said Patent Publication.